In subsea hydrocarbon drilling operations, a Christmas tree may be installed on a wellhead to control the flow of the well. The Christmas tree contains various hub terminations, which are subsequently used to couple the Christmas tree to various manifolds, connectors, jumpers and the like. The hubs include sealing surfaces that need to be protected from the subsea environment prior to the hubs being connected to other manifolds, connector, jumpers and the like. A high-pressure cap is placed onto the hub prior to lowering the Christmas tree subsea to protect the sealing surfaces from the damaging effects that the subsea environment may cause. The use of a high-pressure cap effectively seals the interior of the Christmas tree at atmospheric pressure or the pressure at the surface. The water pressure outside the Christmas tree increases greatly as depth increases when the Christmas tree is lowered subsea. As a result, the water pressure greatly exceeds the pressure inside the Christmas tree when the Christmas tree is, for example, installed at the wellhead on the seabed.
Once the Christmas tree is installed subsea, a running tool is used to remove the high-pressure cap from the hub to allow the hub to be coupled to another manifold or the like. However, because of the large pressure differential across the high-pressure cap, the pressure across the cap must be equalized prior to removing the cap. The high-pressure cap is provided with a valve that, when opened, equalizes the pressure across the cap. In some cases, the valve is a hydraulically-actuated valve and the running tool comprises hydraulic lines that operate the valve. However, such hydraulically-actuated valves are complex and have a large form factor, making it difficult to maneuver the running tool onto the high-pressure cap or to place other manifolds or connectors in close proximity to the hub. Additionally, the height of the high-pressure cap (or distance the cap extends from the hub when installed) may be increased by such valves, requiring connectors to be placed a greater distance away from the hub, which in turn complicates subsequent coupling to the hub. In particular, “wet parking,” which is one advantage of a horizontal coupling system, may be further complicated. Further, hydraulically-actuated valves may need to displace a certain volume of seawater to move from a closed position to an open position, requiring a large amount of force and further increasing the form factor of the valve and associated actuation mechanisms.
Additionally, if a component of the Christmas tree (e.g., valves) malfunctions, the high-pressure cap is the last barrier to the subsea environment. In such a case, the high-pressure cap must contain and relieve pressure from the interior of the Christmas tree. This requires a valve having a large form factor, which includes the disadvantages mentioned above.